(1) Field of the Invention:
This invention relates to spark plugs for use in internal combustion engines, particularly, to spark plugs equipped with electrodes having improved spark wear resistance.
(2) Description of the Prior Art:
In recent years, there is an ever-increasing demand for long-life spark plugs having excellent spark wear resistance with a view toward providing high-performance and maintenance-free internal combustion engines. In order to achieve such prolongation of operating life, it has heretofore been known to improve spark plugs by using a noble metal at discharge-related areas of their center electrodes and ground electrodes made of a heat and erosion resistant metallic material such as a nickel alloy. It has hence been proposed to use pin- or sheet-like tips of a noble metal material such as platinum or a platinum alloy by bonding them in accordance with resistance welding or laser welding. Such pin-like tips however require the noble metal material in a large volume, resulting in a higher manufacturing cost. On the other hand, such sheet-like tips are obtained by stamping circular tips out from blanks of the noble metal material. The circular tips are then bonded. Many stamped blanks are thus resulted, leading to a yield as low as 30% and a cause for the increased manufacturing cost. Noble metal alloys which cannot be readily formed into a pin- or sheet-like shape, for example, Pt-Ir alloys with 25% or more Ir, Rt-Rh alloys with 45% or more Rh, pure Ir and the like have not been used as tips due to their difficult formability although they are expected to improve the spark wear and erosion resistance. Since spark plugs are subjected to a repeated cooling and heating cycle in internal combustion engines, it has been unable to use any noble metal material so long as it tends to develop accelerated cracking and oxidative erosion at the boundary of its bonding with the associated base electrode material by thermal stress caused due to its difference in coefficient of linear expansion from the electrode stock.